This Silvicultural Systems Guidebook will aid you in the process of choosing a silvicultural system. This process includes setting stand-level resource management objectives, stand structural goals and requirements for field data collection, then evaluating silvicultural system alternatives and documenting the silvicultural system prescription to meet the requirements of the Forest Practices Code of British Columbia Act and higher-level plans. The Silvicultural Systems Guidebook promotes consistency in silvicultural systems terminology throughout British Columbia, as a common reference for silvicultural systems definitions and concepts.
Silvicultural systems are an integral part of silviculture prescriptions. They are legislated in the Forest Practices Code of British Columbia Act and the regulations and standards enabled by the Code. The next section of this guide summarizes the sections of the Act relating to the choice and application of silvicultural systems.
This guidebook focuses primarily on the process of choosing a silvicultural system, rather than providing generic standard recommendations for the use of particular silvicultural systems in given ecological or management situations. Definitions of the various systems and features of their application are discussed.
This approach has been deliberately chosen, bearing in mind the variability and complexity of our forest ecosystems and land use patterns. Throughout British Columbia, higher-level plans have been or are being developed for specific areas at the local, regional and provincial level. These higher-level plans, as well as site-specific ecological considerations, vary from place to place. They are key considerations in the process of choosing a silvicultural system.
The choice and application of a silvicultural system for a given forest type must consider many site-specific, ecological, social and operational factors. The choice of the silvicultural system should be customized to the site type, soil, stand structure, silvics of the desired tree species, health and vigour of the stand, and often unique combinations of local forest management objectives. The decision cannot be reduced to simplistic mimicking of textbook techniques that may be applicable to other climates, tree species or ecosystems. In choosing a silvicultural system, the silviculturist must combine these considerations with local knowledge of site and species ecology, regeneration requirements of different tree species, stand development processes, traditional and new harvesting techniques, and the anticipated silvicultural needs of the forests.
A silvicultural system, by definition, encompasses site-level planning, data collection and surveys, regeneration, harvesting and stand-tending treatments over the foreseeable life of a stand. A silviculture prescription or stand management prescription must be consistent with the stated silvicultural system, and will encompass a specific time period for that system.
Readers of this Silvicultural Systems Guidebook are to refer to the Silviculture Prescription Guidebook and other appropriate guidebooks when selecting and implementing a silvicultural system.
The Silviculture Systems Guidebook is divided into five sections:
In addition, two appendixes cover the following subtopics:
The Forest Practices Code of British Columbia Act, section 12(a)(i), mandates that a silviculture prescription (SP) for a forest area to be harvested must specify “a description of the silvicultural system, any harvesting methods and the silvicultural treatments that will be carried out to achieve a free growing stand.” Section 12(b) also specifies that SPs must “be consistent with any forest development plan, higher-level plan and 5-year silvicultural plan.” Section 13(b) states that “A stand management prescription must be consistent with any higher-level plan and 5-year silviculture plan.”
Under section 209(1) and (2)(a) of the Act (Regulations), “the Lieutenant Governor in Council may make regulations respecting silvicultural systems and silviculture treatments,” and respecting “clearcutting, including limiting or prohibiting clearcutting.” Further, regulations can be made respecting “harvesting methods, including limiting or prohibiting the use of a harvesting method” (section 210).
Using appropriate, widely recognized terms for different silvicultural systems, variants and phases helps to clearly communicate these concepts among the people, organizations and agencies that choose, prescribe and implement silvicultural systems. The main function of silvicultural systems terminology is to provide a shorthand nomenclature for the various combinations of post-harvest stand structures and silvicultural treatments, and for how these prescribed structures and treatments will be managed over time.
Silviculture is defined as managing forest vegetation by controlling stand establishment, growth, composition, quality and structure, for the full range of forest resource objectives.
A silvicultural system is a planned program of treatments throughout the life of the stand to achieve stand structural objectives based on integrated resource management goals. A silvicultural system includes harvesting, regeneration and stand-tending methods or phases. It covers all activities for the entire length of a rotation or cutting cycle.
In this guidebook, six major categories of silvicultural systems are recognized: five even-aged systems and one uneven-aged system. Even-aged categories include the clearcut, patch-cut, coppice, seed tree and shelterwood systems. Uneven-aged systems are termed selection systems. Although not silvicultural systems, intermediate cuttings (commercial thinnings and pole harvests) are included in this guidebook as a silvicultural system category for administrative convenience.
A silvicultural system variant further describes the functional attributes of a system. A variant describes the general distribution of cut-and-leave areas or leave-trees on an area over time. The variant indicates whether reserve trees are being retained on-site for long-term non-regeneration objectives, and whether these reserves are relatively uniformly distributed single trees, distinct groups or a mix of both. The silvicultural system variant is the recommended minimum level of description of a silvicultural system for a silvicultural prescription or stand management prescription.
A cut phase of a silvicultural system variant describes the function of a specific harvest to extract merchantable timber and achieve a specific silvicultural treatment. These include regeneration cuts and salvage cuts. The cut phase indicates, to a degree, the timing of the cut within the prescription.
Commonly used terms such as “partial cutting,” “selective logging” or “partial retention logging” are not specific silvicultural systems; they describe harvesting methods. Partial cutting refers generically to stand entries, under any of the several silvicultural systems, to cut selected trees and leave desirable trees for various stand objectives. Partial cutting includes harvest methods used for seed tree, shelterwood, selection and clearcutting-with-reserves systems.
In a silviculture prescription, do not use generic harvesting terms such as “selective cutting” or “partial cutting” to describe a silvicultural system. This section of this guidebook identifies appropriate terms for silvicultural systems and their variants. Remember that each silvicultural system prescription is unique and may not fit the descriptions exactly. However, choose the closest variant based on your silvicultural intent.
Use precise terminology to describe the harvesting phase (e.g., “uniform shelterwood establishment cut with hand-falling and ground-skidding”): the cut phase and harvest method to be employed.
The terminology for silvicultural system categories, variants and cut phases is summarized in Table 1. Table 2 compares such criteria as leave-trees, canopy gap and age structure for the different silvicultural systems.
Characteristics typical of even-aged stands are:
Generally, most or all trees within a unit are clear-felled at one point in time at the end of the rotation. Regeneration occurs within a short period after harvest. Specific regeneration strategies, such as planting, natural regeneration or retention of some advance regeneration, cannot be termed variants of the clearcut system, but simply different regeneration methods in the clearcut system.
With the clearcut system, the opening size and dimensions created (greater than one hectare and greater than two mature tree heights) is generally large enough to limit significant microclimatic influence from the surrounding stand.
The clearcut system has two variants:
Figure 1 represents stand development under a clearcut or patch cut system.
The patch cut system has two variants:
The coppice system has two variants:
Due to the shade-intolerant nature of most of British Columbia’s deciduous or hardwood commercial tree species (including trembling aspen, the species for which this system is most widely used), opening sizes for the coppice system are generally larger than one hectare.
Figure 2 provides an example of stand regeneration and development under a coppice system.
In some cases, the seed or establishment cut is preceded by a preparatory cut, as described for shelterwood systems below.
Figure 3 provides examples of uniform and grouped seed tree system variants.
Uniform seed tree system
In a uniform seed tree system, individual trees are excluded from harvesting and tend to be uniformly distributed throughout the stand unit.
Grouped seed tree system
In a grouped seed tree system, groups of trees are excluded from harvesting. For seed dispersal, the individual groups of trees should be uniformly distributed throughout the stand unit, but an irregular distribution may be necessitated by the natural distribution of the most desirable seed trees on the site.
Shelterwood systems remove successive components of the existing stand in a series of cuttings. Regeneration may be planted, or natural regeneration from seed, or pre-established advance regeneration from the pre-harvest stand. Specifically, the intent of these cuttings is to:
There can be up to three cut phases in a shelterwood or seed tree system:
Preparatory cuts
Establishment, regeneration or seeding cut
Overstorey removal cut(s)
The following sections describe the six different variants of shelterwood systems. Uniform, strip and group shelterwood systems are identified by the spatial arrangement of leave-trees. Irregular, natural and nurse-tree shelterwood systems differ by the timing of overstorey removal. Nurse-tree shelterwood is used to manage different species (often shade-intolerant and -tolerant) in different canopy layers.
Uniform shelterwood system
A uniform shelterwood system consists of individual leave-trees distributed relatively uniformly throughout the stand unit.
Figure 4 represents an example of a uniform shelterwood system.
Strip shelterwood system
A strip shelterwood system involves a series of progressive, usually linear cuts in narrow successive strips, as illustrated in Figure 5.
The basic principles influencing strip orientation include, but are not limited to, minimizing damaging effects on the residual stand (e.g., wind), maximizing shading of cut strips by the uncut areas, and accommodating terrain.
For example, for wind-prone stands, strips in the lee of prevailing storm winds could be cut first, while windward windfirm strips would be left until later stand entries. In areas where drought and excessive daytime heat stress would likely adversely affect regeneration, strips could be oriented east-west to maximize shading of cut strips; in this case, northern strips would be cut first.
Figure 5 presents an example of the strip shelterwood system.
Group shelterwood system
In a group shelterwood system, small openings are created in the stand such that the adjacent trees shelter the new regeneration. The size or density of leave-tree groups will decrease through one or more future stand harvests, until the mature overstorey has been completely removed. Regeneration methods for the final area to be harvested may include natural or artificial regeneration or a combination of both.
Figure 6 illustrates an example of the group shelterwood system.
Irregular shelterwood system An irregular shelterwood system can incorporate characteristics of other shelterwood systems. The key characteristic of the irregular shelterwood is that, although prompt regeneration is an objective, residual trees are left for long periods beyond the regeneration phase (e.g., from 20 per cent of the rotation to several rotations). Residual trees initiate new age classes of regeneration, accumulate wood volume increment and, if desired, achieve non-timber stand objectives.
Due to the protracted retention of leave-trees, the resulting stand is broadly aged and therefore intermediate between an even-aged and an uneven-aged stand. The reserved trees can be left for a defined or indefinite period after the regeneration phase. Both group and uniform patterns of leave-tree retention can be formed.
Figure 7 illustrates an example of the irregular shelterwood system.
Natural shelterwood system
As mentioned earlier, in some natural stands, natural processes have allowed the establishment and growth of understorey trees under a canopy of mature even-aged trees. A two-aged stand such as this is termed a natural shelterwood.
Because acceptable advance regeneration is already present in natural shelterwoods, protection of existing advance regeneration can be the primary regeneration strategy. Potentially, the main stand treatment necessary to regenerate the site is to carefully harvest the mature overstorey and allow the growth of released understorey trees. This overstorey removal strategy is sometimes referred to as a natural one-cut or simulated shelterwood. Overstorey removal is a regeneration treatment that forms one component of a longer-term silvicultural system.
Here are important points to consider before prescribing a natural shelterwood system:
Figure 8 illustrates an example of the natural shelterwood system.
Nurse-tree shelterwood system
A nurse-tree shelterwood system manages different species in two different layers. The tree canopy shelters and provides a more suitable environment for establishment and juvenile growth of the young regeneration. The overstorey can be gradually removed, or removed all at once when the regeneration is developed enough to withstand open site conditions. Where applicable, this system enables shade-intolerant and shade-tolerant tree species to be managed on the same site for a period of time. An example of this system is the managing of a spruce understorey with a lodgepole pine, aspen or paper birch overstorey.
Figure 9 provides an example of the nurse-tree shelterwood system.
Single tree selection system
The single tree selection system is defined as an uneven-aged silvicultural system in which new age classes are created by the removal of individual trees of all size classes, more or less uniformly throughout the stand.
A single tree selection system removes individual trees or small clusters of trees during stand entries, spaced over relatively short specified time periods or cutting cycles. This system creates or perpetuates the multi-layered uneven-aged nature of the stand by creating very small canopy gaps. At each stand entry, designated trees in certain diameter classes are cut, leaving behind acceptable stocking of desirable trees in all diameter classes (with room for adding more trees to each diameter class).
Over the long run, cumulative volume removals from a well-stocked uneven-aged stand managed under the single tree selection system should balance the average periodic volume increment for the stand over the same time. In addition to controlling volume removal, the selection cuts must maintain or enhance stand structure and characteristics to achieve long-term stand-level management objectives.
Single tree selection systems often have been confused with diameter-limit “high-grading” partial cuts, which focus on harvesting the largest, most valuable tree sizes or species in a stand—a “take the best and leave the rest” approach. Legitimate selection systems, as illustrated in Figure 10, strive to maintain or improve forest quality and health through judicious planned cutting in all size classes—“taking the worst first.”
Historically, natural or advance regeneration has been relied upon for selection systems. Planted (artificial) regeneration can also be done, particularly for regeneration of species that may not have a suitable local seed source or reliable seed supply. The specified uneven-aged and uneven-sized structure of the residual stand must meet the objective specified in the prescription.
Figure 11 illustrates an example of the single tree selection system.
Group (and strip) selection systems
The group selection system is defined as a silvicultural system that removes trees in defined groups to create stand openings with a width less than two times the height of adjacent mature trees, and that manages the area as an uneven-aged stand.
A group selection system has potential for stands that are presently either uneven-aged or even-aged. This system is often used for conversion of even-aged stands to uneven-aged stands, where appropriate.
To regulate the stand-level harvest of a stand under group selection, the total area to be harvested from a stand at each stand entry must be determined. This area is based on the length of the cutting cycle, the planned rotation age of the groups and the percentage of the unit available for long-term management. This method of stand-level cut determination is termed area regulation.
As a simple example, in an even-aged stand that has a rotation age of 100 years, 92 per cent of the stand is available for group selection management outside of permanent access structures and group reserves, and the cutting cycle is 25 years. A rotation of 100 years with a cutting cycle of 25 years will support four age classes (25 years, 50 years, 75 years and 100 years old at the end of the fourth cutting cycle). The percentage of the available area that can be harvested at each stand entry is equal to 92% divided by 4 (age classes), or 23% of the stand area per cutting cycle. The openings can be naturally or artificially regenerated.
Under group selection, each opening created is generally small enough to receive significant protection and shelter from the surrounding stand for the whole area of the opening (two tree lengths or less in diameter).
A modification of group selection is strip selection. Narrow strips, of a width less than two times the height of the adjacent mature trees, are progressively cut generally in linear strips distributed throughout the stand. The basic principles influencing strip orientation include, but are not limited to, minimizing damaging effects on the residual stand (e.g., wind), optimizing shading of cut strips by the uncut areas, and terrain considerations.
For example, for wind-prone stands, strips in the lee of prevailing storm winds could be cut first, while windward windfirm strips would be left for later stand entries. In areas where drought and excessive daytime heat would likely affect regeneration adversely, strips could be oriented east-west to maximize shading of cut strips; in this case, northern strips would be cut first.
Figure 12 illustrates an example of the group selection system, and Figure 13 illustrates the strip selection system.
Stand-level reserves are defined as single trees, identified uncut groups or a mixture of both, retained for a defined period or indefinitely to meet objectives other than regeneration.
Reserves can be uniformly distributed single trees or small tree clusters, large well-defined groups, or a mix of two reserve types. Reserve trees are not intended to provide any more than incidental seed or shelter to the regeneration stand and site. Use of reserves can be compatible with any silvicultural system, under appropriate stand and site conditions.
Figure 14. An example of a system with reserves.
Variants of intermediate cuttings include:
Note: Although intermediate cuttings are included with silvicultural systems in this guidebook,
Intermediate cuttings are a stand-tending phase of a silvicultural system. They are different from reproduction or regeneration cuttings, which are part of the regeneration phase. Prescriptions for intermediate cuttings differ from those for most regeneration cuttings in terms of post-harvest stand structural goals. Refer to the Silvicultural Prescription Guidebook for more specific information on prescription requirements for intermediate cuttings.
To achieve higher-level plans and stand-level objectives, reserves may be included in an intermediate cut prescription.